Zengzhi Du,Chunxi Li,Wei Sun,Jianhong Wang*

College of Chemical Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Simulation Real components Diesel hydrotreating True boiling point

A B S T R A C T Computer simulation is a good guide and reference for development and research on petroleum refining processes.Traditionally,pseudo-components are used in the simulation,in which their physical properties are estimated by empirical relations and cannot be associated with actual chemical reactions,as no molecular structure is available for pseudo-components.This limitation can be overcome if real components are used.In this paper,a real component based method is proposed for the simulation of a diesel hydrotreating process by using the software of Unisim Design.This process includes reaction units and distillation units.The chemical reaction network is established by analyzing the feedstock.The feedstock is characterized by real components,which are obtained based on true boiling point curve.Simulation results are consistent with actual data.

1.Introduction

Petroleum refining is used to separate petroleum into fractions,which are further processed to petroleum products by reaction and distillation.In the process,undesirable components are removed from most petroleum products,including diesel,kerosene,fuel oil,gas oil,and w axes,which is essential to public health and environment.The refining process is complicated not only in its unit operations,but also in its components involved.Computer simulation of refining process helps people study the process and its effect on the economy and the environment.For the simulation of refining process,it is necessary to characterize the complex mixtures in a reasonable way.Two approaches are often used,viz.the lumped method[1]and the pseudocomponent method[2].The former is suitable for complex reaction systems,but less accurate for the simulation of distillation.In the second method,pseudo-components are derived from the characterization curve of true boiling point(TBP)and their physical properties are estimated by empirical relations.Hariu and Sage[3] first used the pseudocomponent method for the flash calculation.Eckert and Vanek[4]and Yao and Wang[5]pointed out some disadvantages as follows.

(1)In chemical reactions,no chemical property can be defined and no reaction model can be built for pseudo-components.

(2)Many physical properties of pseudo-components,such as critical properties and acentric factor,are estimated by empirical method.

(3)Some parameters,e.g.binary interaction parameters,cannot be estimated by group contribution,because no molecular structure corresponds to pseudo-components.

(4)It is unsuitable for some product quality in dices in the simulation such as octane number.

(5)It cannot meet the demand of deeper fractions.

In general,the lumped theory is used in reactor models[6–8]and the pseudo-component method is used in distillation simulation[9,10],but neither of them is used to simulate a process with both reaction and distillation.Although the lumped theory can simulate the reaction,only a small number of components are considered,which is far less than that considered in distillation simulation,leading a low accuracy in distillation part.On the other hand,pseudo-components without molecular structure cannot be used to build chemical reaction network to simulate reaction.To overcome this limitation,the real component method has been proposed by Yao and Wang[5],in which the petroleum is characterized with real components,whose physical properties are available in database.

Petroleum is a complex mixture containing a large number of components.With the development of computer science and technology,it is feasible to represent the real petroleum with a huge number of real components.In order to construct a mechanistic model for the simulation of a petroleum processing system,the following are needed.

(1)Physical property data

(2)Rigorous thermodynamic model

(3)Mass balance equation

(4)Energy balance equation

(5)Rigorous calculation of phase equilibrium

(6)Reaction network

In order to build mechanistic model with actual physical property for each component involved,real component method is needed,which is more accurate than pseudo-component method in the modeling and simulation[11],and higher accuracy can be expected in industrial practice[11–13].The real component method has been applied successfully to the simulation of crude oil distillation[4]and the quench system of ethylene plant[14],but has not been used for the whole refinery process with reaction and distillation.The use of real components to characterize reaction and distillation will be more beneficial to the study on petroleum refining process.

In this work,real components are used in the simulation of diesel hydrotreating process,which is mainly used for the removal of organosulfur compounds and organonitrogen compounds,as well as for the saturation of aromatics from feedstock.A mechanistic model and corresponding reaction network are developed based on real components.The diesel hydrotreating process is first described and Peng–Robinson–Stryjek–Vera(PRSV)equation is chosen as the thermodynamic model.The method is applied to determine the group of real components by distillation curve.By analyzing the reaction network,all reactants are identified and added to the group of real components,and there action equations for simulation are determined.The atom balance is also achieved for chemical reactions.The diesel hydrotreating process is simulated by commercial simulation programs of Unisim Design and the results will be compared with actual process data.

2.Real Component Method

The procedure for the selection of real components is shown in Fig.1.First,the overall temperature range in TBP curve is divided into a series of temperature intervals.Each real component is assigned to one of the intervals and an initial group of real components is determined based on TBP curve.Then,by analyzing the reaction network,reaction equations are determined based on the real components,with sulfur and nitrogen compounds added to the reactant group.The final group of real components is determined with other factors,e.g.hydrogen consumption and distillation curve of product.

Fig.1.Procedure for determination of real components.

2.1.Determination of real component group

Test methods for characterizing petroleum components usually include ASTM D86,ASTM D1160,ASTM D2887,and ASTM D3710.ASTM D86 is measured at atmospheric pressure,ASTM D1160 is taken at reduced pressure,and ASTM D2887 and ASTM D3710 are obtained by gas chromatography.Real components are determined based on TBP data.In this simulation,ASTM D86 data are used and converted to TBP data according to following scheme[15].

w here TBP(50)refers to the true boiling point distillation temperature at 50 volume percent distilled,°F,and ASTMD86(50)refers to the ASTM D86 distillation temperature at 50 volume percent distilled,°F.

To determine the difference between cut points,the following equation is used

w here Yiis the difference distillation temperature in TBP,°F;Xiis the difference distillation temperature in ASTM D86,°F;A and B are the parameters as listed in Table 1;and i is the cut point range.

Table 1Parameters A and B

To determine the true boiling point temperature at any percent distilled,Eq.(1)is employed first to calculate the TBP distillation temperature with 50%distilled;Eq.(2)is then used to calculate necessary TBP differences;and following equations are applied to calculate desired TBP distillation temperature.

The unit of temperature is Fahrenheit in Eqs.(1)–(8),which is converted to centigrade by °C=(°F ? 32)× 5/9.

After the TBP curve is obtained,it is cut into a series of temperature intervals,and as elected real component is assigned to a temperature interval.If several real components are in the same temperature interval,the initial real component is used in accordance with the following equation.

w here tr,kis the normal boiling temperature for real component,tm,kis the cutting point temperature in TBP curve,and k is the number of components.

The initial group of real components is thus determined,to which naphthenic and aromatic components can be added if paraffin,naphthenic and aromatic(PNA)composition of petroleum is available.It is difficult to find normal boiling point data of higher hydrocarbon(C30+)in literature,and their physical properties can be calculated by group contribution,such as in[16,17].

For a specific reaction,the normal boiling point of a real component in the reaction shall fall in the range between initial boiling point(IBP)and end boiling point(EBP),

w here Tb,iand Tb,eare the IBP and EBP for TBP curve,respectively,and Tb,ris the boiling point of real component.

Once process parameters are identified,the reactive real components group can be determined according to experimental results in literature,and the data obtained from Eqs.(1)–(9)can be used to simulate distillation process.The final group of real components can be obtained to combine initial and reactive real component groups.

2.2.Determination of feedstock composition based on real components

The composition of feedstock is also an important information for process simulation.According to measured data points,a polynomial describing the TBP curve is built,by which the composition of real components is calculated.

where y is the distillation temperature,x is the cumulative liquid volume percent,aithe is polynomial coefficient,and i is the exponent.

In this polynomial,y equals to the normal boiling point of real component and x is calculated numerically by New ton's iteration method.This polynomial gives cumulative liquid volume percents of all real components at distillation temperature y.The composition of each real component is the difference between two neighboring cumulative liquid volume percents.

3.Diesel Hydrotreating Process

Diesel hydrotreating is one of the most important processes in a modern refinery to produce low sulfur oils,which is a complicated process because of the components,chemical reactions and separation processes involved.In this work we consider a 3500 kt·a-1diesel hydrotreating process with reaction unit and fractionation unit as shown in Figs.2 and 3,respectively.The mixed stream of petroleum and hydrogen is sent to the reactor after preheating,and the product stream goes to the fractionation unit through four separators.In the fractionation unit,low pressure oil from reaction unit is sent to a stripper.The oil from the bottom of stripper is sent to the fractionator,and the refined diesel is obtained as the bottom of the tower.

The feedstock and product specifications of the petroleum are shown in Tables 2 and 3.

In order to simulate the w hole diesel hydrotreating process,light components must be considered.The components of fresh hydrogen coming from catalytic reforming are listed in Table 4.

α is added to modify the original PR EOS by Stryjek and Vera[18],in which an empirical parameter κ is introduced for a better fitting to the vapor pressure of pure component.The detailed expressions for α of component i are as follows.

Fig.2.Flow sheet of reaction unit.

Fig.3.Flow sheet of fractionation unit.

Table 2Feedstock specifications

Table 3Product specifications

Table 4Light components

where κ1is a characteristic parameter of pure component and ωiis the acentric factor.

In this simulation,the conversion reactor is configured based on the amount of reactants and products.By comparing the data in Tables 2 and 3,the conversion of hydrodesulphurization can be obtained.As no nitrogen and aromatic compounds are detected in the products,the conversions of related reactions are assumed to be 100%.The calculated conversions are list in Table 5.

Table 5Conversion for hydrodesulphurization,hydrodenitrogenation and hydrogenation of aromatics

4.Results and Discussion

4.1.Identification of the initial group of real components

The TBP curve is calculated by Eqs.(1)–(8).The TBP distillation temperature at different volume percents is shown in Fig.4.The normal boiling point temperature range is from 111 °C to 385 °C,so the boiling point of real components should be in this range.Cutting ranges are automatically assigned by software.Cutting temperature points are separated by 14–16 °C.According to Eq.(9),normal boiling points of selected real components should be closer to cutting temperature point,as diesel hydrotreating is the second processing step,w here alkane is the main component.The initial group of real components is shown in Table 6.

Fig.4.Distillation curve of TBP and ASTM D86 for feedstock.□ TBP;■ ASTM D86.

Table 6Initial group of real components

4.2.Determination of chemical reaction network

Hydrotreating is a process where hydrogen is used to remove sulfur and nitrogen from diesel streams and to saturate aromatics.Oxygenated compounds and olefin content are negligible in diesel stream[20],so the reaction of deoxygenation and olefin saturation are not considered in this simulation.In this section,three main reaction groups in diesel hydrotreating process are analyzed,viz.hydrodesulphurization(HDS),hydrodenitrogenation and hydrogenation of aromatics[21].

4.2.1.Hydrodesulphurization

The sulfur compounds in crude oil may be classified into two groups[20].One is non-thiophenic sulfur compounds including mercaptan(R–SH),thioether(R–S– R′)and disulfide(R–S–S–R′),and the other is thiophenic sulfur compounds including thiophene,benzothiophene,dibenzothiophene,etc.The feedstock is mixed by straight-run diesel and cracking diesel,so the content of nonthiophenic sulfur compounds is negligible.Based on the TBP curve of feedstock,the main sulfur compounds are benzothiophene and dibenzothiophene[20].The reaction network for sulfur compounds is shown in Fig.5.

The Mayoress, a long thin stick of a woman, enraged17 at the insult done to her husband, seized his free arm and tore at it with all her might, with the only result that she too was forced to swell18 the procession

Fig.5.Chemical reaction network of hydrodesulphurization.

4.2.2.Hydrodenitrogenation

The nitrogen compounds include heterocyclic aromatic compounds and nonheterocyclic organonitrogen compounds[21].The former includes pyridine,quinoline,and indole,and the latter includes anilines,amines and nitriles.Based on TBP curve of the feedstock,the main nitrogen compounds are pyridine,quinoline and indole,while the nonhetercyclic organonitrogen compounds are not considered in the reaction network since their amount is very small[22].The reaction network for nitrogen compounds is shown in Fig.6.

Fig.6.Chemical reaction network of hydrodenitrogenation.

4.2.3.Hydrogenation of aromatics

The aromatic compounds are petroleum mixtures,mainly composed of monoaromatics,diaromatics,triaromatics,and some polycyclic aromatics[23].The content of polycyclic aromatics is negligible in the feedstock of diesel hydrotreating[20],so only the mono-,di-,and triaromatics are considered in the simulation.Based on the IBP and EBP from TBP curve,the representative aromatic compounds are toluene,ehtylbenzene,naphthalene and anthracene[21,22].The reaction network for aromatic compounds is shown in Fig.7.

Fig.7.Chemical reaction network of the saturation of aromatics.

4.2.4.Reactive components of reaction network

According to the above analysis for chemical reaction network,the reactive group of real components is shown in Table 7.

Table 7Reactive group of real components

4.3.Determination of the composition of real components

Based on measured data in Table 2,four TBP distillation curves are obtained by polynomial fitting,as shown in Fig.8,with quadratic (a),cubic (b),4th order (c),and 5th order(d)polynomials.The value of R2of 5th order polynomial is close to 1 and the trend is consistent with measured value,so the composition of each real component is calculated by the following polynomial in this study.

The volume composition of real component is listed in Table 8.The datacalculated with Eq.(19)are shown in column 4.The contents of reactants cannot match the feedstock specifications in Table 2.A range of contents of sulfur compounds,nitrogen compounds and aromatic compounds in diesel hydrotreationg process has been reported[20].According to the literature,benzothiophene/dibenzothiophene mass ratio is about 1:3.2,pyridine/indole/quinoline mass ratio is about 1:8:14,and bicyclic aromatic/polycyclic aromatic mass ratio is about 5:1.Thus the contents of reactants can be confirmed,and they are in accord with actual total sulfur content,total nitrogen content and total aromatic content in Table 2.The compositions of other real components need to be adjusted in order to agree with TBP curve.The final compositions of real components are listed in column 5.

In Fig.9,these real componentsare used to characterize the distillation curve of feedstock.The calculated TBP curve is in a good agreement with TBP curve,which is converted from actual ASTM D86 curve data.

4.4.Simulation of diesel hydrotreating process

The diesel hydrotreating process is simulated by Unisim Design.Results for some important simulated data are listed in Table 9.The calculated results using real components for the simulation of w hole diesel hydrotreating process match actual process data quite well.The deviations are acceptable for engineering application.

After the simulation for diesel hydrotreating with reaction unit and fractionation unit,the distillation data of the refined diesel from the bottom of the fractionator are calculated.Fig.10 show s that the calculated TBP curve of refined diesel can match that converted from actual ASTM D86 curve data.The feedstock can be characterized by more real components,if PNA composition of petroleum is available.

In this work,the diesel hydrotreating process is also simulated without HDS.Fig.11 show s a comparison of distillation data for converted data,calculated data with and without HDS.It demonstrates that the presence of HDS does not affect the distillation curve of refined diesel.How ever,HDS reactions are essential for environmental protection.The sulphur content is 4937 μg·g-1without HDS.It is much higher than 50 μg·g-1in National Standard for Automobile Diesel Fuels(IV)of China[24].Thus HDS reactions have to be included in the diesel hydrotreating process.

EOS models are usually used to calculate the properties for hydrocarbon mixtures[11],among which cubic equations of state,e.g.Peng–Robinson and Soave–Redlich–Kw ong(SRK),are used frequently.The calculated distillation curves are plotted in Fig.12 based on PRSV EOS and SRK EOS.Both can simulate the diesel hydrotreating process with good accuracy.

5.Conclusions

Fig.8.Fitting for TBP distillation curve.○measured value;— fitting curve.

Table 8Composition of real component

A real component method for a diesel hydrotreating process is proposed.The group of real components,determined based on TBP curve,is used to characterize feedstock.After establishing reaction network,reactive components are added to the group of real components.The PRSV EOS and the SRK EOS are employed to calculate thermodynamic properties.Both models can be used to simulate the diesel hydrotreating process.The comparison of the simulation results with the measured data proves that the proposed approach is effective in the simulation of diesel hydrotreating process.Thus these real components can be used to describe chemical reaction and simulate similar processes,and it is possible to be used for process optimization and equipment design.Related reaction can be further investigated when more kinetic information is available.

Table 9Comparison of simulation results of diesel hydrotreating process and actual data

Fig.10.Comparison of converted and calculated TBP distillation curves for refined diesel.○converted value;■calculated value by real components.

Fig.11.Comparison of converted and calculated TBP distillation curves with and without HDS for refined diesel.○ converted value;▼ calculated value by real components with HDS;△calculated value by real components without HDS.

Fig.12.Comparison of converted and calculated TBP distillation curves by PRSV EOS and SRK EOS for refined diesel.○ converted value;▼ calculated by PRSV EOS;△ calculated by SRK EOS.

Nomenclature

A parameter defined in Eq.(2)

a attraction parameter

B parameter defined in Eq.(2)

b van der Waals covolume

k number of component

P pressure,kPa

Pccritical pressure,kPa

R universal gas constant,J·mol?1·K?1

r real component

T absolute temperature,K

Tbboiling temperature,K

Tb,eend boiling point for true boiling point curve,°C

Tb,iinitial boiling point for true boiling point curve,°C

Tb,rboiling point of real component,°C

Tccritical temperature,K

V molar volume,m3·kmol?1

X distillation temperature in ASTM D86,°C

Y distillation temperature in true boiling point,°C

Z compressibility factor

α scaling factor defined in Eq.(16)

κ1characteristic pure component parameter

κiempirical parameter defined in Eq.(17)

ωiacentric factor